A fission of uranium-235 releases on average 1.86 neutrons, some of which trigger
fission in nearby nuclei and some of which escape without triggering fission.
If a sphere of uranium-235 is small then most of the neutrons escape before
triggering fission and the sphere doesn't blow up.
If the sphere is large then most of the neutrons trigger more fission,
a chain reaction occurs and the sphere blows up. The threshold for a chain reaction
is the "critical mass".

Two pieces of uranium-235, each with less than a critical mass, are
brought together in a cannon barrel.

If the uranium is brought together too slowly, the bomb fizzles.

If you bring two pieces of uranium-235 together too slowly, a chain reaction
begins in the near side of each piece, generates heat, and blows the two pieces
apart before they can come completely together. Only a small amount of uranium
undergoes fission and this is referred to as a "fizzle". Using gunpowder and a
cannon is fast enough to properly detonate uranium and this is technologically
easy to do.

Plutonium detonation

Plutonium is more difficult to detonate than uranium. Simply bringing two pieces
together, no matter how fast, results in a fizzle. To detonate plutonium you have to
shape it as a sphere and implode it, which is technologically difficult.

In World War 2 the U.S. produced enough uranium for 1 bomb and enough plutonium for
2 bombs. One of the plutonium bombs was tested in the "Trinity" test before being
used in the war, and the second bomb was dropped on Nagasaki. The uranium bomb was
dropped on Hiroshima without previously being tested.

When Hans Bethe, a physicist on the Manhattan project, was asked why they didn't test
the uranium bomb he replied "Because we were perfectly sure it would work".

Natural Uranium is .72% Uranium-235 and 99.3% Uranium-238. Only Uranium-235 undergoes
a chain reaction and so it has to be separated from the Uranium-238. Several methods
exist for doing this. In World War 2 the isotopes were separated magnetically
with calutrons. Gas diffusion and centrifuges can also be used.

Centrifuge separation of uranium-235

UF6

UF6

Light blue: uranium-235. Dark blue: uranium-238

Centrifuges

Uranium is converted to gas form by forming uranium hexafluoride
(HF6). HF6 is a gas above 64 Celsius. In a centrifuge,
the lighter uranium-235 concentrates at the center and the heavier uranium-238
concentrates at the edge.

"Coulomb energy" is the product of the charges of the two reactants, in units
of proton charge. The lower the energy, the easier it is to fuse the nuclei.
This can be seen with
the Rutherford
scattering simulation.